Venturi-type and blower air/gas mixing systems with radiant fiber matrix burners are well known, as may be seen from the Final Report, January 1987, entitled High Efficiency Hydronic Heating Unit, Gas Research Institute, FIG. 4, etc. (see also its preceding Gas Research Institute TECHNOLOGY PROFILE, October 1985). That Final Report in Table 1, entitled "Burner Characteristics," mentions that NO.sub.x can be lowered with higher excess air and/or burner inserts. That prior mentioning was, however, expressly limited to flameholder type of burners and did not provide any teaching to this effect for the porous type burners.
Indeed, European Patent Application 0 157 432, entitled "Radiant surface combustion burner," by Shell Internationale Research Maatschappij B. V., inventors: D. A. C. McCausland et al, published 9 Oct. 1985, pointed out that NO.sub.x values are much higher in free-flame mode of operation, than in the radiant surface combustion mode.
This was further confirmed in U.S. Pat. No. 5,205,731, by J. J. Reuther et al, issued Apr. 27, 1993 to Battelle Memorial Institute, for "Nested-Fiber Gas Burner," and herewith incorporated by reference herein. In particular, that patent pointed out that porous, radiant burners emit only about ten percent of the nitrogen oxides, NO (NO+NO.sub.2), of ported, blue-flame burners, but that port loading (energy released per unit area per unit of time, such as in kWm.sup.-2 or in Btu/sqin-hr) of conventional radiant, porous-matrix burners was only on the order of two to five percent of ported blue-flame burners.
Both the above mentioned European patent application and United States patent operate with sintered nested-fiber gas burners having high porosity, whereby higher port loading becomes possible, given as being between 100 to 1000 kWm.sup.-2 for porosities of 60% to 90% in the European application, and as between 800 to 5300 Btu/sqin-hr for porosities of 80% to 89% given in the Battelle patent. Especially port loadings at the higher porosities contrast very favorably to possible port loadings of 100 to 400 kWm.sup.-2 or some 1000 Btu/sqin-hr for radiant surface combustion burners using fiber porous elements.
Both the above mentioned European application and the Battelle patent expressed preference for non-woven steel fibers containing chromium and aluminum, such as an alloy containing iron, chromium, aluminum and yttrium or FeCrAlY, or iron-chromium-aluminum electrical-resistance wire.
Both the above mentioned European application and the Battelle patent reported low nitrogen oxide emission ratios, such as between 12 and 24 ppmv at 200 and 600 kWm.sup.-2, respectively, or less than 20 ppm at port loadings of from 800 to 5300 Btu/sqinhr, accompanied by carbon monoxide emission of less than 50 ppm. By contrast, the above mentioned European application mentions nitrogen oxide emissions of between 150 and 250 ppmv in free-flame modes of operation.
In particular, that European application mentions a free-flame mode of operation for its porous burner as occurring above 2000 kWm.sup.-2, and a transition region between 1000 kWm.sup.-2 and 2000 kWm.sup.-2 wherein both surface combustion and free-flame combustion were observed.
Against this background, the above mentioned Battelle patent, as mentioned above, reported low nitrogen oxide and carbon oxide emission ratio at high port loadings of up to 5300 Btu/sqin-hr or some 2400 kWm.sup.-2 through a retraction of the early portion of the blue flame (there called "leading edge of the flame front") into the top layer of its nested-fiber burner. In particular, the Battelle patent teaches connecting valve means to the inlet of the sintered nested-fiber burner in order to (1) control the admission of a combustible gas and oxygen mixture from a source to that burner and (2) insure that the pressure of the gas and oxygen mixture admitted to that burner locates the leading edge of the flame front of that gas/oxygen mixture, which is ignited within the mat of that sintered nested-fiber burner, to the inside of that nested-fiber burner; that is, between the inner and outer surfaces of its nested-fiber mat.
It is thus most remarkable that significant lowering of nitrogen oxide and carbon monoxide emissions was reported to have been achieved through adjustment of a simple valve in the combined combustible gas and oxygen supply line, that cannot provide any variation of the supplied oxygen or air relative to the supplied gas. Against this surprising background, the test results reported in the above mentioned European application indicate a relatively low upper limit of 600 kWm.sup.-2 for low nitrogen oxide emission from its porous burners.